The present application claims the priority of Japanese Patent Application No. 2000-316902 filed in Japan on Oct. 17, 200, the entire contents of which is hereby incorporated herein by reference.
1. Field of the Invention
This invention relates to an absolute sensor for detecting an absolute rotational angular position of a rotational shaft.
2. Description of Related Art
As the conventional absolute sensor for detecting an absolute rotational angular position of a rotational shaft, there is known one that has a reduction mechanism constituted by spur gears. The sensor of this type typically comprises a reduction mechanism constituted by a combination of spur gears, a high speed rotational shaft for inputting a high speed rotation into the reduction mechanism, a low speed rotational shaft which is transferred a low speed rotation obtained via the reduction mechanism, and encoders for detecting a rotational angle or rotational speed of the high and low speed rotational shafts, respectively. Based on the respective outputs of the encoders, the absolute rotational angular position of the low speed rotational shaft, for example, can be detected, in which the resolution of detection corresponds to the reduction ratio of the reduction mechanism.
The conventional absolute sensor of this type, however, has the following defects. First, since the reduction mechanism is constituted by a plurality of spur gears, the high speed rotational shaft at the input side cannot be aligned coaxially with the low speed rotational shaft at the reduced rotational speed output side. Second, it is difficult for the reduction mechanism constituted by the combination of a plurality of spur gears to obtain a high reduction ratio, so that a high resolution sensor cannot be realized. Third, a plurality of gears must be connected to achieve a desired reduction ratio, which is against downsizing of the sensor.
An object of this invention is, in view of the above points, to provide an absolute sensor in which input side and output side rotational shafts can be aligned coaxially.
Another object of this invention is to provide a small absolute sensor which has a high resolution.
Another object of this invention is to provide an absolute sensor in which a reduction ratio thereof is set a power of two so that output signals of encoders thereof can be easily processed by a computer system.
In order to solve the above and other objects, there is provided an absolute sensor which comprises a high speed rotational shaft, a planetary gear mechanism for reducing a high rotational speed delivered from the high speed rotational shaft, a low speed rotational shaft which is aligned coaxially with the high speed rotational shaft and is delivered reduced speed rotation from the planetary gear mechanism, a high speed side encoder for detecting a rotational speed or rotational angle of the high speed rotational shaft, and a low speed side encoder for detecting a rotational speed or rotational angle of the low speed rotational shaft.
The planetary gear mechanism is constituted to have a fixed sun internal gear, a front stage planetary gear, a rear stage planetary gear which rotates integrally with the front stage planetary gear, and an output sun internal gear engaged with the rear stage planetary gear.
The front stage and rear stage planetary gears are rotatably supported by an eccentric shaft portion formed on the high speed rotational shaft and are maintained in a condition engaged with the fixed sun and output sun internal gears, and the output sun internal gear is connected to the low speed rotational shaft.
The absolute sensor of this invention detects an absolute rotational angle of the low speed or high speed rotational shaft based on outputs of the high speed side and low speed side encoders.